With recent advancement of digital technologies, electronic hardware such as portable information devices and home information appliances have been developed to provide higher functionality. For this reason, there have been demands for an increase in a capacity of a nonvolatile memory device, reduction in a write electric power in the memory device, reduction in write and read time in the memory device, and longer life of the memory device.
Under the circumstances in which there are such demands, it is said that there is a limitation on miniaturization of the existing flash memory using a floating gate. Accordingly, in recent years, a novel resistance variable nonvolatile memory element using a resistance variable layer as a material of a memory section has attracted an attention.
As shown in FIG. 24, the resistance variable nonvolatile memory element basically has a very simple structure in which a resistance variable layer 504 is sandwiched between a lower electrode 503 and an upper electrode 505. Upon application of predetermined electric pulses between the upper and lower electrodes, the nonvolatile memory element switches to a high-resistance state or to a low-resistance state. By corresponding these different resistance states to numeric values, respectively, data is stored. Because of such a simple structure and operation, further miniaturization and cost reduction of the resistance variable nonvolatile memory device are expected. Since switching to the high-resistance state or to the low-resistance state occurs in an order of 100 nsec or less in some cases, the resistance variable nonvolatile memory element has attracted an attention because of a potential of a high-speed operation, and a variety of proposals therefor have been made.
For example, Patent document 1 discloses a resistance variable nonvolatile memory element in which metal ions are caused to travel into and out of a resistance variable layer 504 to attain a high-resistance state and a low-resistance state by applying voltages between an upper electrode and a lower electrode, thereby storing data. Also, as disclosed in Patent document 2, there is known a resistance variable memory in which crystalline and non-crystalline states of a resistance variable layer are changed using electric pulses to switch its resistance states (phase change memory).
In addition to the above, there are many proposals for resistance variable nonvolatile memory elements using metal oxides for the resistance variable layer 504. It is considered that these nonvolatile memory elements are operable with a mechanism different from aforesaid mechanisms. It is said that oxygen in the metal oxide layer used as the resistance variable layer migrates in response to electric pulses, causing resistance switching (detail of the mechanism is not made clear yet).
The resistance variable nonvolatile memory elements using such metal oxides are classified into two major kinds depending on the material used for the resistance variable layer. Patent document 3 or the like discloses one kind of resistance variable nonvolatile memory element using perovskite materials (Pr(1-x) CaxMnO3 (PCMO), LaSrMnO3 (LSMO), GdBaCoxOy (GBCO), as the resistance variable layer.
The other kind is resistance variable nonvolatile memory elements using binary transition metal oxides. Since the binary transition metal oxides have a very simple composition and structure as compared to aforesaid perovskite materials, composition control and layer deposition in manufacturing are relatively easy. In addition, the binary transition metal oxides have an advantage that they are relatively highly compatible with a semiconductor manufacturing process. For these reasons, the binary transition metal oxides have been vigorously studied recently. For example, Patent document 4 or Non-patent document 1 discloses NiO, V2O5, ZnO, Nb2O5, TiO2, WO3, and CoO as the resistance switching material. Patent document 5 discloses resistance variable nonvolatile memory elements using as the resistance switching materials, sub-oxides (oxides having a composition outside a range of a stoichiometric composition) of Ni, Ti, Hf, Nb, Zn W, Co, etc. Furthermore, Patent document 6 or Non-patent document 2 discloses an example, in which a structure formed by oxidating a surface of TiN to form a crystalline TiO2 film of a nanometric order is used as the resistance variable layer.
In addition to the above, Patent document 7 discloses so-called a one time programmable memory which uses titanium oxide and tantalum oxide (Ta2O5) as the resistance switching material and is capable of writing only once.